This invention relates to a DC motor speed control system and, in particular, to an improved speed control system which is little susceptible to influence from load fluctuation.
Heretofore, there have been provided motor speed control systems adapted to supply a full-wave rectified current through a semiconductor element with a control electrode to the motor, one of which control systems comprising a charging circuit which is connected through a variable current setting means to a full-wave rectifying circuit, a switching circuit which is connected with the charging circuit and, when the charging voltage in the charging circuit reaches a predetermined value, is made conductive to discharge the charging circuit thereby to supply the discharge current to the semiconductor element as a trigger current, and a detecting means for detecting the counter electromotive force induced in the motor, whereby the conduction phase of the semiconductor element is controlled in response to the detected counter electromotive force so as to stabilize the rotation speed of the motor in spite of load fluctuation. A typical example of a speed control system of this kind has been disclosed in U.S. Patent Application Ser. No. 811,625, now U.S. Pat. No. 4,168,455 entitled "Motor Speed Control System" by Katsuji Soeda et al, filed June 30, 1977, and assigned to the common assignee with the present applicant.
In this system, an automatically variable resistor means, such as a transistor, which is connected with the counter electromotive force detecting means is further provided so that the detected counter electromotive force is supplied as a bias voltage to the transistor whereby the charging current to the charging circuit is shunted by means of the transistor. Thus, the counter electromotive force is converted to the conduction current of the transistor so as to control the charging rate of the charging circuit in dependence on the counter electromotive force, whereby the conduction phase of the switching element or Diac is controlled in response to the counter electromotive force.
However, this system requires the automatically variable resistor means converting the counter electromotive force to the current, thus causing the construction to be complicated.